Over the course of 10 years, Dr. Beth Tarini and a team of researchers studied papers and letters written by Wilder, newspaper articles about Mary's illness and data on blindness and infectious disease in the late 19th century, CNN reports.

Tarini discovered that in Pioneer Girl, Wilder's unpublished memoir, there is no reference to Mary contracting scarlet fever before going blind. "She never says scarlet fever. She never says rash," Tarini told CNN.

Then Tarini unveiled what would lead her to discover a different diagnosis. In a letter Wilder wrote to her daughter, Rose, she referenced Mary suffering from "some sort of spinal sickness." The letter goes on to mention that Mary was told by specialist in Chicago that the "nerves of her eyes were paralyzed and there was no hope."

But why would Wilder write that her sister suffered from scarlet fever? Researchers think perhaps Wilder and her editors felt the illness would be more relatable to her readers.